Fluorocarbon elastomers are synthetic elastomeric polymers with a high fluorine content--see, for example, W. M. Grootaert et al., Fluorinated Elastomers, 8 KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY 990-1005 (4th ed. 1993). Fluorocarbon elastomers, particularly the copolymers of vinylidene fluoride with other ethylenically unsaturated halogenated monomers such as hexafluoropropene (C.sub.3 F.sub.6) have become the polymers of choice for high temperature applications, such as seals, gaskets, and linings. These polymers exhibit favorable properties against the exposure to aggressive environments such as solvents, lubricants, and oxidizing or reducing agents. Additionally, these polymers can be compounded and cured to have high tensile strength, good tear resistance, and low compression set.
Presently used curing agents for fluoroelastomers include aromatic polyhydroxy compounds, such as polyphenols, used in combination with certain vulcanization accelerators such as ammonium, phosphonium, or sulfonium compounds. U.S. Pat. Nos. 4,882,390 (Grootaert et al.), 4,912,171 (Grootaert et al.) and 5,086,123 (Guenthner et al.), for example, describe these compounds.
In accordance with conventional curing processes, desired amounts of compounding ingredients and other conventional adjuvants or ingredients are added to unvulcanized fluorocarbon elastomer stock and intimately admixed or compounded therewith by employing any of the usual rubber mixing devices such as Banbury mixers, roll mills, or other convenient mixing device. The components and adjuvants are distributed throughout the fluorocarbon gum during milling, during which period the temperature of the mixture typically will not rise above about 120.degree. C. The curing process typically comprises either injecting (injection molding) the compounded mixture into a hot mold or pressing (compression molding) the compounded mixture in a mold, e.g. a cavity or a transfer mold, followed subsequently by an oven-cure (post cure).
Many conventional fluoroelastomer compositions tend toward "scorching" behavior, i.e., the premature crosslinking or partial cure of the composition when exposed to elevated temperatures or conditions of high shear. This scorching behavior particularly is pronounced when the fluoroelastomer is injection molded, wherein scorching is characterized by a premature cure initiation occurring prior to and during injection of the compounded composition into a mold. The point of cure initiation for injection-molded fluoroelastomers may be defined as the time after which the compounded fluoroelastomer is subjected to injection molding conditions (i.e., upon introduction into an injection barrel at a temperature above approximately 70-90.degree. C. and/or while injecting the compound into the mold under high shear at temperatures between about 180 and 200.degree. C.) when the curing compound begins to gel or harden. Such a change in physical properties, particularly the corresponding viscosity increase, can greatly reduce processing efficiency by hindering the ability to inject the compounded mixture into a mold. Scorching phenomena also produce high levels of waste product; because a cured fluoroelastomer is very difficult to reprocess, any fluoroelastomer that cures outside the mold cavity must usually be discarded.
Fluoroelastomer compositions that are formulated for high crosslink density (e.g., to achieve high modulus or low elongation) show low elongation at break, and hence articles having complex geometric profiles often crack or tear upon demolding when allowed to fully crosslink inside a mold upon injection molding or press curing. The internal and external defecting of such articles has a self-evident detrimental effect on the properties of the cured product, and where a high percentage of articles must be rejected because of such defects, the overall efficiency of a manufacturing process can be greatly compromised.